1. Field of the Invention
The present invention relates to an improvement of a control apparatus for a drive system of a hybrid vehicle.
2. Description of Related Art
There is known a hybrid vehicle drive system including: a differential device which comprises a first differential mechanism and a second differential mechanism and which comprises four rotary components; an engine, a first electric motor, a second electric motor and an output rotary member which are respectively connected to said four rotary components; and a plurality of coupling elements. JP-2013-224133 A1 discloses an example of a hybrid vehicle transmission system configured to switch the hybrid vehicle drive system to a selected one of a plurality of vehicle drive modes, according to a selected one of different combinations of operating states of the coupling elements.
It is known that the hybrid vehicle drive system constructed as described above has a so-called “mechanical locking point” at which an amount of an electric energy transmitted through an electric path between the first and second electric motors is minimal as a result of zeroing of an operating speed of the first or second electric motor. At this mechanical locking point, a transmission efficiency of the hybrid vehicle drive system has a maximal value. However, the prior art control apparatus for the hybrid vehicle drive system is not configured to implement a vehicle drive mode switching control while taking account of the mechanical locking point, so that the transmission efficiency cannot be effectively improved. This problem was first discovered by the present inventor in a process of intensive studies in an effort to improve the performance of the hybrid vehicle.